The present invention relates to the field of Chemical Mechanical Polishing (CMP). More particularly, the present invention relates to methods and apparatus for chemical mechanical polishing of substrates, such as semiconductor substrates, on a rotating polishing pad in the presence of a chemically and/or physically abrasive slurry, and providing fresh supply of slurry onto the surface of the substrate which is mounted on the polishing pad while the substrate is being polished. Additionally, the present invention includes a pad conditioning apparatus to condition the polishing pad while the polishing pad is being used to polish semiconductor substrates. Additionally, the present invention includes a new slurry delivery system where multi-component slurries can be used that can be metered very accurately during slurry flow and which completely eliminates the use of the conventional peristaltic pump.
Chemical Mechanical Polishing is a method of polishing materials, such as semiconductor substrates, to a high degree of planarity and uniformity. The process is used to planarize semiconductor slices prior to the fabrication of semiconductor circuitry thereon, and is also used to remove high elevation features created during the fabrication of the microelectronic circuitry on the substrate. One typical chemical mechanical polishing process uses a large polishing pad that is located on a rotating platen against which a substrate is positioned for polishing, and a positioning member which positions and biases the substrate on the rotating polishing pad. Chemical slurry, which may also include abrasive materials therein, is maintained on the polishing pad to modify the polishing characteristics of the polishing pad in order to enhance the polishing of the substrate.
The use of chemical mechanical polishing to planarize semiconductor substrates has not met with universal acceptance, particularly where the process is used to remove high elevation features created during the fabrication of microelectronic circuitry on the substrate. One primary problem which has limited the used of chemical the polishing pad is difficult. Providing a fresh supply of slurry to all positions of the substrate is even more difficult. As a result, the uniformity and the overall rate of polishing are significantly affected as the slurry reacts with the substrate.
The polishing process is carried out until the surface of the wafer is ground to a highly planar state. During the polishing process, both the wafer surface and the polishing pad become abraded. After numerous wafers have been polished, the polishing pad becomes worn to the point where the efficiency of the polishing process is diminished and the rate of removal of material from the wafer surface is significantly decreased. It is usually at this point that the polishing pad is treated and restored to its initial state so that a high rate of uniform polishing can once again be obtained.
In the conventional approach, the wafer is held in a circular carrier, which rotates. The polishing pads are mounted on a polish platen which has a flat surface and which rotates. The rotating wafer is brought into physical contact with the rotating polishing pad; this action constitutes the Chemical Mechanical Polishing process. Slurry is dispensed onto the polishing pad typically using a peristaltic pump. The excess slurry typically goes to a drain, which means that the CMP process has an open loop slurry flow. In addition, the conventional approach uses orbital motion where there is a relative motion at any point of the wafer that poses severe problems of non-uniformity across the die and across the wafer in addition to problems of planarity. Also, the conventional approach uses and dispenses with an excessive amount of slurry that adds significantly to the processing cost. There also is no method for exactly controlling slurry flow. The present invention addresses and solves the indicated problems. Since both the wafer and the polishing pad are rotating there exists a velocity differential across the wafer. This velocity differential wafer polishing uniformity and planarity suffer across the die and across the wafer. This limits the application of the conventional CMP approach especially in Shallow Trench Applications, copper damascene, etc., which are involved in sub-quarter micron technology modes.
FIG. 1 shows a Prior Art CMP apparatus. A polishing pad 20 is affixed to a circular polishing table 22 which mechanical polishing in the semiconductor industry is the limited ability to predict, much less control, the rate and uniformity at which the process will remove material from the substrate. As a result, CMP is labor intensive process because the thickness and uniformity of the substrate must be constantly monitored to prevent overpolishing or inconsistent polishing of the substrate surface.
One factor, which contributes to the unpredictability and non-uniformity of the polishing rate of the CMP process, is the non-homogeneous replenishment of slurry at the surface of the substrate and the polishing pad. The slurry is primarily used to enhance the rate at which selected materials are removed from the substrate surface. As a fixed volume of slurry in contact with the substrate reacts with the selected materials on the surface of the substrate, this fixed volume of slurry becomes less reactive and the polishing enhancing characteristics of that fixed volume of slurry is significantly reduced. One approach to overcoming this problem is to continuously provide fresh slurry onto the polishing pad. This approach presents at least two problems. Because of the physical configuration of the polishing apparatus, introducing fresh slurry into the area of contact between the substrate and rotates in a direction indicated by arrow 24 at a rate in the order of 1 to 100 m RPM. A wafer carrier 26 is used to hold wafer 18 face down against the polishing pad 20. The wafer 18 is held in place by applying a vacuum to the backside of the wafer (not shown). The wafer carrier 26 also rotates as indicated by arrow 32, usually in the same direction as the polishing table 22, at a rate on the order of 1 to 100 RPM. Due to the rotation of the polishing table 22, the wafer 18 traverses a circular polishing path over the polishing pad 20. A force 28 is also applied in the downward or vertical direction against wafer 18 and presses the wafer 18 against the polishing pad 20 as it is being polished. The force 28 is typically in the order of 0 to 15 pounds per square inch and is applied by means of a shaft 30 that is attached to the back of wafer carrier 26. Slurry 21 is deposited on top of the polishing pad 20.
FIG. 2 shows a typical Prior Art slurry delivery system. Slurry 21 of uniform chemical and mechanical composition is contained in the slurry vat 34 from where the slurry 21 is pumped by the diaphragm pump 36 in direction 38. The peristaltic pump 40 deposits controlled and intermittent amounts of slurry 21 onto the polishing pad 20 while the balance 44 of the slurry that had been pumped by the diaphragm pump 36 is returned to the slurry vat 34. The rate at which the slurry 42 is provided by the two pumps 36 and 40 can be under control of conditions of operation and environment such as type of surface being polished, rate of rotation of either the wafer 18 and/or the polishing table, etc.
U.S. Pat. No. 5,688,360 (Jairath) shows cylindrical and conical polishing pads.
U.S. Pat. No. 5,709,593 (Guthrie et al.) shows a slurry delivery system and slurry wiper bar.
U.S. Pat. No. 5,785,585 (Manfredi et al.) discloses a polishing pad conditioner with radical compensation.
U.S. Pat. No. 5,792,709 (Robinson et al.) shows a polishing pad disk.
U.S. Pat. No. 5,782,675 (Southwick) discloses an apparatus to recondition a polishing pad.
U.S. Pat. No. 5,679,039 (Talieh) discloses a polishing pad with grooves to deliver slurry.
U.S. Pat. No. 5,775,983 (Shendon et al.) teaches a conical roller to condition the polishing pad.
The present invention teaches an in-situ slurry distribution and concurrent pad conditioning process and apparatus. The novelty of the present invention is that the polishing pads are mounted on a cylindrical platform that consists of a pad/core arrangement, instead of the conventional flat platform on which the polishing pads are placed.
The cylindrical pad has motion in the X-Y-Z directions; the cylindrical pad in addition has rotational motion. The novelty of the present design consists of as unique pad/core design with the polishing pads mounted on the surface of the core. Evenly spaced openings are provided within the pad/core assembly for the location of slurry ports.
The center of the core is hollow; slurry is pumped through the center of the core and exits the core through the slurry ports to the polishing pads and the pad conditioners.
The present invention in addition incorporates a new slurry delivery arrangement. The slurry, which can consist of a combination of more than one type or composition of slurry, is pumped in the conventional manner (for instance using diaphragm pumps) and flows through an orifice-flow meter where the multi-component slurries are combined and pumped through a single tube mixing coil. The actual mixing of the different slurries occurs within the mixing coil. The mixed slurry flows through a rotating driver that
In this way, a constantly renewed supply of fresh slurry can be provided to the wafers which are being polished thus eliminating previously experienced problems associated with stationary or used slurry. This aspect of the present invention is of particular importance for the polishing of metal surfaces.
Using this approach of the present invention, the slurry can be metered very accurately unlike the slurry flow of conventional applications where the peristaltic pump causes a great deal of irregularities in the flow of the slurry. In addition, the present invention allows for the complete elimination of the peristaltic pump.
As part of the present invention, a pad conditioner disc used. This disc is of the same shape as the pad/core assembly and fits snuggly around this assembly. The pad conditioner conditions the polishing pads at the same time that the polishing operation takes place. The friction between the pad conditioner and the pad/core assembly can be varied during and as part of the polishing process thus further adding a parameter of control for the polishing operation.
The method used for increasing the friction or pressure between the pad conditioner and the pad/core assembly can be of a number of designs, for instance air-actuated cylinders can be used for this purpose. This allows for very accurate control of this application parameter.